In general, an engine includes a piston, a connecting rod, and a crankshaft. The piston is linearly reciprocated in a cylinder under the airtight condition and the linear movement of the piston is converted into a rotational movement of the crankshaft through the connecting rod.
Chemical energy of fuel is converted into heat energy (pressure of gas) through combustion in a mixer and expansion pressure of gas pushes the piston downward while rotating the crankshaft, thereby generating mechanical energy (power) for performing work.
Current combustion technologies of the vehicle generate a great amount of exhaust gas, thereby causing environmental pollution.
The exhaust gas signifies gas exhausted to the exterior through an exhaust pipe after it has been burned in the cylinder. The exhaust gas mainly consists of harmless materials, such as H2O, N2 and CO2. However the exhaust gas also includes harmful materials, such as CO, HC, Nox, lead oxides and carbon particles.
Herein, CO, HC and Nox relate to environmental pollution so that it is necessary to reduce the content of CO, HC and Nox.
Blow-by gas is introduced into a crankcase by passing through a gap formed between the piston and the cylinder, so it is called “crankcase emission”. The blow-by gas mainly consists of unburned HC (70 to 95%). The remaining portion of the blow-by gas is a mixture including combustion gas and partially oxidized gas.
If the blow-by gas exists in the crankcase, an internal wall of the engine is eroded and the quality of engine oil is degraded. For this reason, the blow-by gas is conventionally exhausted to the exterior through ventilating the crankcase. However, since the exhausted blow-gas contains a relatively large amount of HC, an apparatus for purifying HC is separately provided.
CO is colorless and odorless gas generated when fuel is incompletely burned. If CO is absorbed in a human body, CO may be mixed with hemoglobin carrying oxygen in a bloodline, so that oxygen may not be sufficiently fed into each part of the human body. If an amount of CO absorbed in the human body exceeds a predetermined level, a CO-addiction may occur.
In general, if people stay for one hour under the environmental condition of air containing 0.15% of CO, the life of these people may become threatened. This has been proven through the CO-addiction caused by CO generated from a briquette used at home.
Compounds consisting of carbon C and hydrogen H are called “hydrocarbon”. HC is contained not only in the exhaust gas, but also in the blow-by gas or fuel evaporation gas. From among HC exhausted to the exterior from the vehicle, about 60% of HC is exhausted through the exhaust pipe, about 25% of HC is exhausted through the crankcase as a blow-by gas, and about 15% of HC is exhausted as an evaporation gas.
HC having low density may slightly stimulate respiratory organs of the human. However, in an extreme case, the HC may harm a mucous membrane or eyes of the human.
Since nitride compounds include NO, NO2, N2O, etc., they are called “Nox”. Air includes 79% of nitrogen, so that the nitrogen cannot be easily oxidized even if stable combustion is performed. When the nitrogen makes contact with air in the combustion chamber under the high temperature and pressure condition, the nitrogen is oxidized, thereby forming the Nox. The Nox harms eyes of people and degrades the function of the lungs while causing the photochemical smog. The smog is a compound word of “smoke” and “fog”. The exhaust gas contains nitride compounds including 95% of NO2 and 3-4% of NO.
Hydrocarbon or nitride oxide exhausted from vehicles, factories or power plants may not directly form the photochemical smog. That is, the photochemical smog is formed when the hydrocarbon or nitride oxide repeats the photochemical reactions in the atmosphere with strong solar light (ultraviolet ray). At this time, materials exerting the bad influence upon eyes or respiratory organs of the human are secondarily created in the form of the smog.
In general, (PB(CH3)4) or (PB(C2H5)4) is added to gasoline for the vehicles in order to obtain a high octane level, so lead compounds are detected from the exhaust gas of the vehicle. Since (PB(CH3)4) and (PB(C2H5)4) are toxic materials, if the (PB(CH3)4) exhausted in the form of oxidized lead through the combustion procedure is absorbed in the human body through air, the respiratory organs and muscular system of the human may be damaged. For this reason, lead-free gasoline having no (PB(CH3)4) is currently fed to the engine of the vehicle.
Preferably, a theoretical mixture ratio (14.7 weight percent of air:1 weight percent of fuel) presenting the highest combustion efficiency, an economical mixture ratio (16 weight percent of air:1 weight percent of fuel) and a maximum output mixture ratio (12.5 weight percent of air:1 weight percent of fuel) are used as a mixture ratio for a high-performance engine.
The above mixture ratios are suitable for improving engine performance and the combustion speed may significantly vary depending on the mixture ratios. There exist the following relationships between the mixture ratio and an amount of toxic gas contained in exhaust gas.
1. An amount of Nox is reduced and an amount of HC is increased in a rich mixture.
2. An amount of CO and HC is reduced and an amount of Nox is increased in a lean mixture.
3. An amount of Nox and CO is reduced and an amount of HC is increased in a super lean mixture.
Regarding the relationship between the engine temperature and toxic gas, if the engine has a low temperature, fuel cannot be easily atomized and the mixture makes contact with a cooled intake manifold or a cylinder wall so that a part of gasoline is coagulated in a liquid state or a particle sate. Thus, a rich mixture is employed when the engine has the low temperature. In this case, an amount of air in the mixture is insufficient so that CO is generated. In addition, since the combustion temperature is lowered, an amount of Nox is reduced and an amount of unburned HC is increased.
In contrast, if the engine has a high temperature with cooling water having a temperature of 89 to 90° C., evaporation of fuel is promoted so that the engine can be economically operated. However, if the engine has an excessively high temperature, various problems, such as overheat, detonation or pre-ignition, may occur. Thus, the combustion temperature may excessively rise, so that an amount of Nox may increase.
In a case of sudden deceleration, a throttle valve is rapidly closed so that strong vacuum is instantly created in the manifold. As a result, an amount of intake air is reduced and an amount of gasoline is increased, thereby causing a super rich mixture ratio while lowering compression pressure of the cylinder.
For this reason, the combustion temperature is lowered, an amount of CO is increased due to incomplete combustion and an amount of HC is increased because a quenching zone is formed.
A wall of a combustion chamber having a relatively low temperature, a crevice volume formed between a cylinder and a piston and a small gap formed in a valve may not be influenced by flames so that incomplete combustion may occur in the above areas. Such an incomplete combustion area is called a “quenching zone”.
Regarding the relationship between engine load and toxic gas, if the engine operates with a low speed, the engine idly rotates to drive the vehicle with a reduced speed. In addition, if the engine operates with a low speed and low load, the mixture becomes rich, compression pressure of gas becomes reduced, and the combustion speed of the mixture becomes lowered. Accordingly, CO is generated due to reduced pressure and incomplete combustion. In addition, since the gas temperature is lowered, the quenching zone is thickly formed, thereby generating HC. Therefore, the toxic gas is exhausted even if the engine normally operates with low load.
When the engine operates with a low speed, a part of the mixture is uselessly exhausted upon a valve overlap, thereby generating HC.
In contrast, when the engine operates with high load, advanced timing is employed so that the gas temperature rises, thereby improving the combustion efficiency. However, this may cause creation of HC although an amount of CO is relatively reduced.
In order to purify the toxic exhaust gas, a catalytic converter has been proposed. According to the catalytic converter, a mixture of Pt and Rh is attached to a substrate installed at an inner portion of a case of the catalytic converter so as to purify the exhaust gas. Herein, Pt is mainly used for the oxidization action of CO and HC, and Rh is mainly used for the reduction action of Nox.
However, the above catalytic converter may optimally operate only when the mixture having the mixture ratio in the vicinity of the theoretical mixture ratio is burned and exhausted. Thus, the engine must be controlled such that it has the mixture ratio in the vicinity of the theoretical mixture ratio, causing technical problems.